Should I return my African Dwarf Frogs?

Exercise reasonable caution, and you (and they) should be fine. I believe all reptiles and amphibians have salmonella, it's one of their gut bacteria, like all mammals have E. coli. But, as long as you keep everything adequately clean, it shouldn't be a problem.

Since you do have a kid sister in the stick-everything-in-her-mouth age range, I'd suggest that you take special care to make sure there's nothing portable that you can drip water on when you're changing the water in the tank. Temporarily move any keys, etc to the table or something when you're changing your frogs' water. And, of course, wash your hands after handling them or putting your hands in their water or the like.

But, the salmonella concentration in the water will be pretty small (at least, if you keep it properly cleaned and filtered), so the chances of your sister getting enough of the bacteria to actually make her sick are pretty small, unless she actually, say, sticks her hands in the tank, then directly into her mouth, or something. Which is unlikely. Make sure the older one knows to wash her hands after doing *anything* with the frogs, if she ever does so, both by example (always wash your hands after handling them) and by telling her directly.

But, really, don't worry about it too much. Especially since... as long as your family is healthy, they're very unlikely to get very sick even if they do somehow get salmonella, it's a very minor risk to anyone with a decent immune system. Older people... often have at least somewhat compromised immune systems, so that's why it was such a problem for your grandmother, probably.

As ingrid said, it's nothing to worry about. Not only do many people own frogs, but all reptiles can carry salmonella and think about how many people own reptiles!!

As long as you wash your hands after handling them, you'll be fine :)

I wouldn't worry about it. Thousands of people own them, there are very few reported cases of salmonella